,- ANIMAL DRAUGHT CULTIVATIONS

-

Frank Minns

1. Development of a high-lift harness and lightweightplough system

Background to the 'high-lift'

systemDraught anima! cultivation systems have

been the subject of much re~earch for

several thousand years. Until recently. it

has been conducted by fanners on a trial

and error basis. resulting in some

remarkably efficient harne~s/implement

combinations. More recent inve~tigation~

have concentrated on implement de~ign

without con~idering the interaction

between animal and implement. incontra~t to the detailed inve~tigation~

which have been made into the hitching

of tractor-pulled implements.

Mouat and Coleman (1954) showed

that the draught, H, of a cultivation

implement is a function of the vertical

force, ~ acting on it and the angle, a, at

which it is being pulled. The relationship

may be stated in the form of a Tillage

Implement Draught Equation (TIDE) as:

H = V/tan a

or, in words:

The drau,l;Jht (~f a cultivationimplement varies directly ~vith the

effective vertical.force (e.v..f) actingon it and inversel.}' with the tangent(~f the an,l;Jle a1 which it i.\" being

pulled.Although this relationship is very simple,

its implications are quite protound. Theyhave been explored in some detail by Inns

(1990) with consequences which

sometimes conflicted with the

perceptions of designers and other

experts, but were generally in accordance

with the experience of users.

Frank Inns, FIAgrE, was formerly

Professor of Agricultural Machinery

Engineering at Silsoe College and is

now a Consultant in Small Farm

Mechanisation. His address is: 53

Alameda Road, Ampthi", Bedford,

MK45 2LA, tel. 01525402508.

Experimental verification of therelationship between ploughdraught and angle of pull

It seemed necessary to demonstrate thaithe TIDE did give a true prediction of therelationship between plough draught and

the angle of pull, a. -consequently fieldexperiments were conducted at the Centrefor Tropical Veterinary Medicine(CTVM). Edinburgh (Inns & Krause.1995) using a donkey and a 15 cm

mouldboard plough weighing 18 kg. Thepredicted relationship was confirmed. Itwas then decided to face the challenge ofdesigning a harness/plough system tomatch the draught capability of a singledonkey, i.e. about 200 N to 250 N. Thischallenge is relevant to currentcircumstances in many develop;ng

countries in Africa and elsewhere, wherethere is a shortage of traditional draughtanimal!; for small-scale farming anddonkeys are increasingly recognised asbeing an underutilised resource.

Preliminary trials were made alCTVM using a breastband harness fittedwith a hipstrap to vary the angle of pullbetween about 200 to 350 (liS sho""'11 ill

Landwards, Spring 199812

provide an angle of pull of 300approximately, as shown in Figure 4. Thebreastband harness is suitable for a singledonkey, horse or mule -it is possible tode~ign an alternative high-lift harness foroxen. By standardising the angle of pullit has been possible to design a dedicatedlightweight plough weighing 8 kg and

expectations and further trials wereundertaken on-fann in conjunction withfarmers in Tanzania (see TitlePhotograph), with results as shown inFigure 3. In general, it appears thatplough draught is halved when the angleof pull is increased from 20° to 30°,making all the difference between a no-

Fig. 1 plain breastband harness -angle of pull fixed at about 20degrees.

Hip strapShoulder strap

Harness

~

~ h .Hake anac ment IXXnt

I~~

Depth of wortf

Fig. 4 Harness design: typical high-lift harness (breast band type)suitable for donkeys and other equines. This harness gives an angleof pull of about 30 degrees, or slightly more.

go situation and one which the donkeycan cope with throughout a working day.Design of a high-lift harness anddedicated lightweight ploughFollowing from the above experiences,a simple and cheap 'high-lift' breastbandtype harness has been designed to

suited to manufacture in small localworkshops. The harness and plough haverecently (January 1998) been evaluatedfor use in Bolivia. Results have beenexcellent when working with eitherdonkey or horse as shown in Figures 5and 6. Versions of the harness andplough have been manufactured locallyin Bolivia.

Fig. 2 Breastband harnessmodified by additian of anadjustable hip strap -angle ofpull can be varied from about 20to 3S degrees.

Figures J and 2) and a 11.5 cm ploughweighing about 12 kg, which wasdesigned to cope with the variation inpull angle. Results confirmed

1000

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~Q)

c

I

-400Ia

~

"t)

~0 200

a.

Advantages of the high-lift

systemAdvantages of the high-lift system,leading to a high degree of user-friendliness, include:.improved plough efficiency (greater

proportion of useful work);.reduced draught load on the animals

(or more useful work for the same

draught level);.easier adjustment (skids or wheels

are not needed);.simple harness (cheap, easily made

locally); and.lighter plough (cheaper, more easily

transported and handled).The improvement in plough

efficiency arises from the combinationof reduced plough weight and increaseduplift on it as a result of the steeper angleof pull. These combine to reduce the loadcarried by the wheels of a wheeledplough or, in the case of a wheel-lessplough, the load reduction occursbetween the underside of the plough andthe soil. Rolling resistance at the wheels

015 20 25 30 35 40

An 9 I e of pull, a (de 9 rees)

Fig. 3 Variation of draught with angle of pull: results of field trials in

Scotland and Tanzania.

13Landwards, Spring 1998

800

600

~AL

DRAUGHT <:ULTIVATIONS

~;I

awkward for the ploughperson to handle.Why suffer these difficulties when ..1lighter plough will avoid the adjustmentinvolved and will also be cheaper andmore efficient? Many locally made

ploughs (Pakistan, Turkey, Jordan, elc)are relatively light and work very easilyand effectively: thus it is the practitionerswho have a better understanding of thefundamental relationship between ploughweight and draught. The TillE endorsesthe practitioners' viewpoint.

.A lighter plough has inherentlyless draught and is more user-

friendly.

-:

:;c~7-:.";:';;-

':7

This conflicts with a

long-held belief that

"the weight of the

plough has

comparatively little

effect on its draught"

(Young, 1784,

quoted by Mouat &

Coleman, 1954) -an

opinion which is still

shared by many

advisers and larger

scale manufacturers.

What are the facts?

, Older P lou g hsFig. 5 High-lift harness and lightweight plough at were very heavy and

work with donkey, Copinota, Bolivia. the e.v.f. would be

very large, with

correspondingly high draught, unless the

plough's weight were partially

counteracted by upward-acting support

force(s). This was done by supporting

most of the weight on a large 'sole' or

'slade' -a long horizontal plate

underneath the plough -or by wheels,skids or other devices. If the support force

is large enough (not necessarily easy to

achieve) the e.v.f. can then be reduced to

.1 value which will result in an acceptable

level of draught -~ I but much of it

will be parasitic

draught caused

I by friction on the

sole, or rolling

resistance from

I the wheel(s),

which do no.useful work on

the soil.

Over the

years the use of

improvedmaterials and

design .tllowed

~ the plough to be

m.tde lighter.

The support

force did notI\:\:J 10 Ol: ~l' grcat, the sole orthc plough

oliid hc made smaller, the par.lsitic

Jr.tught was reduced and hence the

plough bec.tme more efficient. It also

became easier to adjust, ifit h.ld been well

designed, because it was no longer

necessary to provide a very 1.lrge support

force.

Work animals can cope with a heavy

plough if it is carefully adjusted to

develop a large enough support force, but

unnecessary weight makes it more

is reduced and/or the frictional resistance

underneath the plough. Plough efficiencyis improved by reducing these parasitic

components of draught. There are

preliminary indications that share wearis also reduced.

2. Some myths andrealities relating to animalpowered ploughsThe investigations and design work

-\:-,~~rt,:

Plough weight and penetrationIt is often argued that weight is necessaryto get the plough to penetrate to its

working depth. The TIDE suggests amore subtle and effective approach -perhaps the e.v.f. is too small? What isthe reason? Almost invariably, it is notbecause the weight (main downwardforce) is too little, but because supportforces are too big. Unwanted supportforces can arise under the point of a wornshare, if there is insufficient clearance('pitch', 'suck' or 'down suction') behindthe share or sometimes because the loweredges of the mouldboards are pressingdown hard on the soil and providingunwanted support along their bottomedges (the mouldboards of ridging bodiesare often badly shaped). Look for shinysurfaces underneath the implement -auseful indicator of areas where the soilmay be providing too much ",upporl.

.Lack of penetration is a/mo.\'1always a question of too milch.\'upport, not a lack of weight.

,t'

h~'-:"\'~.?l /-~ .'*'~~\;~~:..,...,- -,' '..'

'.-:~~f.~-.,.

.~~~~~

-..

, ..-"

Fig. 6 High-lift harness and lightweight plaugh at

work with horse, Cochabamba, Bolivia.

reported ahove involved considcrahlc )"'" ',- practical work. carefully undertaken.

observed and thought about. Some light l;'--

may be thrown on existing animal (

draught controversies as a spin-off. l

:g~;.:~~ ~:l:::.

The influence of plough weight on

plough draughtFarmers and other practitioners of

animal-powered ploughing often makestatements to the effect that "this ploughis

too heavy for my animals. (to pull)".

The influence of angle of pull on

plough draughtThe TIDE suggests that (he dr.lught or a

plough will be reduced as (he .Ingle of

pull is increased, if the e.v.f. acting on it

rem.lins constant. This effect W.IS noted

more th.ln 150 years .Igo by Pusey ( 1840)

who obscrved the advantagcs gaincd frl)m

a stceper angle of pull, but found it

difficult to find a scientific explan~tion.

Although some succes,-;ful harnes'idesigns feature a steep angle of pull. ithas been adopted through ad h()c

development rather than by intent. The

concept is indeed disputed by some

specialist advisers.It has been shown in theory and

practice that plough draught can be very

significantly reduced by using a steeper

14 landwards, Spring 1998

~ ~~~~~:;,;;,~,"",i(l\ .\\3Cn~e{\ y- ~\SeOa~ -=-- .nu\'

\~O\ ..:I \.CO{{eC\) \

O{\~\(l3

Pulling point

~\.i(\~

JJ --~;::..~~~;:;::, ~

Fig. 7 The effect of raisin~, the poinlt of attachment of I~he pull chain.

Showing how a raised att4~chment IPoint reduces the angle of the lineof pull but raises its line ot: action. The new line of pull creates aclockwise turning effect about the f,orce centre, causin~, the plough torun nose down.

Working adjustments to the plough-setting the plough into workAs is well known, the basic principle isto attach the pull chain (or rope) to thehake at the point where the line of pulland the hake adjustment coincide. If thechosen attachment point is not quite

angle of pull. This fact can be used bydesigners and operators to improve

system efficiency..For optimum results, and ease ofcontrol, the plough and harnessmust be designed as an integratedcolnbination.

'If-'

"'-

(/,~-:

Working adjustments to the plough-changing the depth of workTraining manuals often state that toincrease the depth of ploughing the pullchain should be attached at a higher pointon the hake. This does reduce the angleof pull (very slightly) and shouldtherefore increase the draught and depthof work, but unfortunately it also changesthe location of the line of pull so that itno longer passes through the force centre('centre of resistance') of the plough butpasses above it to produce a nose-downturning effect (Figure 7).

The manuals then advise that thenose-down movement should becounteracted by lowering the nose wheelinto contact with the ground to make theplough run level again. But this producestwo contradictory effects. First: the angleof pull is reduced -draught and depth ofwork should be increased. Second: thewheel develops a support force from thesoi I -this acts to reduce the e. v.f. andhence also the draught and depth of work.The net effeCt on depth of work is zero,but the wheel and its bearings are putunder load unnecessarily, leading toexcessive bearing wear. Ploughefficiency is reduced due to rollingresistance of the wheel.

The correct method of adjusting depthof work is by changing the angle of pulland going through the initial setting

procedure again using the hakeattachment to fine tune the plough forlevel running.

~

The two illustrations to the Jeft show breast band harne!sses with rigid(usually stitched) connections to the !,houlder straps -the breastband isheld vertical and the top edge tends to dig into the donl<ey's chest. Theright hand illustration show's a brea~;tband harness witl, adjustableshoulder strap which is free! to pivot at its connection to the breastband-these arrangements allow the breast band to be set tOI the correctheight and to take up its most comfortable angle.

Working adjustments to the plough-conclusionSo are farmers justified in their suspicion-and often their rejection -of specified

adjustment procedures? Are they makinga logical response? The answer isprobably; "Yes", for two reasons. First:once a correct adjustment is achieved,there is little reason for changing it if theline of pull is not changed -this is onlylikely to happen if the length of the pullchain is altered or if different draughtanimals are used which differsignificantly in height. Second: theadjustment procedures they have beentaught were probably confusing and mayhave been incorrect. The nosewheel, ifused in the way which is often writtenabout and advised, causes a rollingresistance which reduces efficiency. Itsonly useful function is to assist in turningand transporting the plough but, if this isnecessary, the plough is too heavy

Fig. 8 Preferred arrangemE~nt for a breastband harnes5:, shown on the

right.

Working adjustments to theplough -backgroundFarmers are often criticised for not usingthe hake attachment points or other formof regulator correctly, or even forthrowing the regulator away andwrapping the pull chain round the ploughbeam in a permanent position. Butperhaps it is the critics who are at faultfor not giving good (and oftenconflicting) advice on adjustment of thepoint of attachment to the hake? Thereare two matters to consider: setting theplough into work and changing the depthof work.

correct this will be shown up in work -.the plough will run either nose up or nosedown. This can be corrected by a 'finetuning' adjustment at the hake.

It is suspected that some ploughs arepoorly designed, with the hitch pointspositioned incorrectly so that there is nohitch point available on the line of pulljoining the harness pulling point and theplough's force centre. In such cases, theplough cannot be adjusted to run withcorrect balance and the ploughperson willhave to struggle continuously to hold theplough in a reasonable working

alignment -this can be an extremely tiringprocess. Correct design is essential for

joyful ploughing.

Landwards, Spring 1998 15~

ANIMAL DRAUGHT CULTIVATIONS

anyway!.The nose wheel (or skid) that isusually fitted to swing ploughs is aheavy, expensive and unnecessarydistraction which, if the plough iswell designed, is a hindrance to itsproper adjustment and control.

Harness designA breast band harness was chosen for theexperimental work referred to abovebecause, compared with the mainalternatives, it is simpler, cheaper andmore easily made locally and is thereforemore user-friendly and accessible to smallscale farmers.

One particular feature of the harnesswas observed to be particularly helpfulto the donkey's comfort in use and caneasily be incorporated into otherbreastband designs. The ability to adjust

the breastband to fit comfortably on theanimal's chest -not its neck! -was greatlyaided by making the shoulder strapadjustable for length and by connectingit to the breastband by a fitting (a ring inthis case) which allowed them to pivotrelative to each other. This allowed thebreastband to be adjusted for height andto sit at an angle to give maximumcomfort.

Many existing harness have a rigidstitched connection between thebreastband and the shoulder strap (andany backstrap(s) which may be fitted) sothat the breastband is held vertically atthe chest. The top edge of the breastbandthen digs hard into the animal -

particularly if the breastband is made ofrigid material such as transmissionbelting, which appears to be a favouredmaterial. This is the case with the donkeyshown ploughing in the ntle Photographobviously not desirable.

The shoulder strap and breastbandform the basic assembly used in abreastband harness and Figure 8illustrates some of their desirable and lessdesirable features as mentioned above.

The

AcknowledgementsThe author wishes to thank: The LeverhulmeTrust for the award of an Emeritus Fellowshipwhich supported much of the experimentalwork reported in this paper; The DouglasBomford Trust for providing support for travelto Ethiopia to present a paper and demonstratethe high-lift system at an InternationalWorkshop held at Debre Zeit 5-9 May 1997;Project Equipment Ltd, Oswestry and ChapelStudio, Penzance for manufacture of ploughs;farmers and many others whose interest,comments and discussion have contributed tothe development of the high-lift cultivation

system.

Trust

Assisting thefurtherdevelopment of

AgriculturalEngineering andMechanisation

through sponsoredresearch, andcareer development

programmes.

References

Inns F (1990). The mechanics of animaldraught cultivation implements, part I,chain pulled implements. Agric. Engt; 45

(I): 13-17.Inns F, (1996). Matching tillage implements

to draught animal potential. World AnimalReview, 86:40-49.

Inns F, Krause P (19950. Experiments toinvestigate the effect of angle of pull on the

draught of a chain-pulled swing plough.Draught Animal News, No. 22; 2-6.

Mouat G, Coleman F (1954). Tillageimplements. Temple Press, London.

Pusey P (1840). Experimental inquiry ondraught in ploughing. J. Royal AgriculturalSociety of England, I; 218-243.

Further informationfrom:The Secretary,Douglas Bomford Trust,16 The Oaks, Silsoe,Bedford MK45 4ELTel: 01525 861144.

landwards, Spring 199816